This invention relates generally to methods and apparatus for use in drying sections of coating machines in which one or both sides of a web have been treated with a coating agent or size, or in drying sections of paper machines. More particularly, the invention relates to web drying methods and apparatus wherein a web to be dried is pressed in direct contact against the outer mantle of a drying cylinder and then carried by a wire over a lead cylinder with the web situated on the outer side of the wire, whereupon the web is then carried by the wire to the next drying cylinder.
It is well known to use sizing and coating devices in the finishing of paper for coating one or both sides of a web with a coating agent or size. When a web is coated in this manner, the web itself becomes moist and subject to stretching.
In a coating machine, a tension or pulling force is exerted on the web as it moves through various groups of cylinders. To provide the required pulling force, the web is usually pressed against the surface of the cylinder by means of a wire. In such a case, it is common to employ either a twin-wire draw, a so-called UNO RUN (.TM.) draw, or a wire draw, either at the upper or lower side of the cylinder group. The cylinders are usually heated, and are occasionally cooled. It is also known in the art to employ so-called drawing-roll groups whose only function is to draw the web through the machine without any drying effect.
As the running speed of coating machines increases, a sufficient web drawing effect often cannot be obtained by means of conventional techniques since the permeability to air of coated paper is low, so that with increasing web speed, the moving surfaces produce such a high positive pressure in the nip between the cylinder and incoming web that the web begins to slip over the cylinder surface. At higher speeds, the web also begins to flutter and, in the case of an UNO RUN-(.TM.) draw, the web becomes detached from the wire.
The same problems discussed above also occur in multi-cylinder drying sections of paper machines. It will therefore be understood that, although the following description is in connection with an application of the invention to coating machines, the method and apparatus of the invention are also suitable for use in other applications, such as in drying sections of paper machines, to solve precisely the same problems discussed above. Furthermore, by the present invention, it is possible to reduce fluttering of the web and the possibility of detachment of the web from the wire. The invention also facilitates the threading of the web in paper machines.
In the case of UNO RUN draws, there is a movement towards the use of cylinder groups in which the cylinders that are in contact with a wire are perforated suction rolls. In such cylinder groups, the cylinders can be situated in horizontal rows, or in vertical stacks, or in combinations of the same.